refactor(compression): tidy in-place compaction path (simplify pass)

Parallel 3-reviewer cleanup of the in-place compaction code. Findings applied:

- perf: in-place mode no longer pre-flushes current-turn messages. The flush
  ran INSERTs that the immediately-following replace_messages(compressed)
  DELETE+reinsert discarded -- pure wasted writes per compaction. The
  current-turn tail survives via the compressor's compressed output
  (protect_last_n), not the flush. Verified no data loss; rotation still
  pre-flushes (its old session row is preserved, so the flush is real there).
- quality: hoist the two shared post-write steps (update_system_prompt +
  _last_flushed_db_idx = 0) below the if/else -- they ran in both branches
  against agent.session_id. Removes the easiest divergence bug.
- quality: compute the compaction-boundary locals (_old_sid, _is_boundary,
  _boundary_parent) ONCE instead of recomputing locals().get('old_session_id')
  and the "_old_sid or agent.session_id or ''" chain three times.
- quality: initialize compacted_in_place up front and assign
  agent._last_compaction_in_place directly, dropping the fragile
  locals().get('compacted_in_place') reflection.
- reuse: parse the in_place config flag with utils.is_truthy_value (the
  project's canonical truthy coerce) instead of a hand-rolled
  str().lower() in {...} (agent_init already imports from utils).

Dropped as false positives / out of scope: gateway getattr of agent internals
(established session_id pattern), dual result-dict carry (mirrors history_offset
etc.), stringly-typed "compression" (codebase-wide convention, no constant).

Behavior-preserving: 7 in-place tests (incl. 2 new flush-guard tests) + 26
rotation/boundary/persistence/command tests green; mutation check confirms the
durable-replace guard still binds (removing replace_messages fails the test);
ruff clean. Added test_in_place_skips_redundant_preflush /
test_rotation_still_preflushes to guard the perf change.

agent/agent_init.py

@@ -50,7 +50,7 @@ from agent.tool_guardrails import (
 from hermes_cli.config import cfg_get
 from hermes_cli.timeouts import get_provider_request_timeout
 from hermes_constants import get_hermes_home
-from utils import base_url_host_matches
+from utils import base_url_host_matches, is_truthy_value
 
 # Use the same logger name as run_agent so tests patching ``run_agent.logger``
 # capture our warnings.  (run_agent.py also does
@@ -1344,9 +1344,9 @@ def init_agent(
     # parent_session_id chain, no `name #N` renumber). See #38763 and
     # agent/conversation_compression.py. Consumed by compress_context(), not the
     # compressor, so it rides on the agent.
-    compression_in_place = str(
-        _compression_cfg.get("in_place", False)
-    ).lower() in {"true", "1", "yes"}
+    compression_in_place = is_truthy_value(
+        _compression_cfg.get("in_place"), default=False
+    )
 
     # Read optional explicit context_length override for the auxiliary
     # compression model. Custom endpoints often cannot report this via